The nuclear factor of activated T cells (NF-AT) is an inducible, lymphoid-specific transcription factor that is essential for expression of the IL-2 gene upon T cell activation (For a review, see Ullman, K. S., Northrop, J. P., Verweij, C. L., Crabtree, G. R. (1990) Ann. Rev. Immunol. 8, 421-452). By cell fractionation and reconstitution experiments, NF-AT was shown to be assembled in the nucleus of activated T cells from a T cell-specific component that is preexisting before activation and an inducible nuclear component (Flanagan, W. M., Corthesy, B., Bram, R. J., and Crabtree, G. R. (1991) Nature 352, 803-807). The preexisting component of NF-AT (here designated NF-AT.sub.p) was subsequently identified in hypotonic extracts of unstimulated T cells by its ability to bind specifically to an oligonucleotide corresponding to the distal NF-AT sequence from the murine IL-2 gene promoter (Jain, J., McCaffrey, P. G., Valge-Archer, V. E., and Rao, A. (1992) Nature 356, 801-804). In addition, it was shown that the inducible nuclear component of NF-AT consists of Fos and Jun proteins, (Jain et al., supra).
Assembly of NF-AT in the nucleus requires two intracellular signals, the activation of protein kinase C and an increase in cytosolic free calcium, both of which are provided by activation of T cells through the T cell antigen receptor. Activation of protein kinase C is necessary for transcriptional induction of Fos and Jun genes (Jain, J., Valge-Archer, V. E., and Rao, A. (1992) J. Immunol. 148, 1240-1250). An increase in intracellular calcium is necessary for the appearance of NF-AT.sub.p in the nucleus, presumably reflecting its translocation from the cytosol (Flanagan, W. M., Corthesy, B., Bram, R. J., and Crabtree, G. R. (1991) Nature 352, 803-807). The immunosuppressive drugs cyclosporin A (CsA) and FK506 block induction of NF-AT by interfering with the calcium-dependent appearance of NF-AT.sub.p in the nucleus. CsA and FK506, when complexed with their specific intracellular binding proteins (cyclophilin and FK506 binding protein, respectively), potently inhibit the activity of the calcium- and calmodulin-dependent phosphatase, calcineurin (Klee, C. B., Draetta, G. F., Hubbard, M. J. (1987) Adv. Enz. 61, 149-200); Fruman, D. A., Klee, C. B., Bierer, B. E., Burakoff, S. J. (1992) Proc. Natl. Acad. Sci. USA 89, 3686-3690), towards peptide substrates (Liu, J., Farmer, J. D., Jr., Lane, W. S., Friedman, J., Weissman, I., and Schreiber, S. L. (1991) Cell 66, 807-815) Based on these results, it has been proposed that NF-AT.sub.p in the cytosol may be a target for calcineurin, either directly as a substrate or indirectly via a phosphatase cascade (Schreiber, S. L., and Crabtree, G. R. (1992) Immunology Today 13, 136-142).